This invention relates to line interface circuits and line drivers and in particular to a line interface circuit or line driver which includes a novel impedance matching arrangement which is suitable for use in a digital subscriber line (xDSL) modem as well as methods of operating the same.
Line drivers such as Asymmetric Digital Subscriber Line (ADSL) drivers, Very High Speed Digital Subscriber Line (VSDL) drivers and others generally called in the art xDSL line drivers (where xe2x80x9cxxe2x80x9d represents the type of technology) are known. Some such prior art arrangements are discussed in, for example, Bill Schweber xe2x80x9cAnalogue front ends bridge the xDSL to real world chasmxe2x80x9d, EDN, Apr. 1, 1999; E. Nash, xe2x80x9cLine driver design for broadband communications applicationsxe2x80x9d, Electronic design, Dec. 1, 1997; and from M. Steffes, xe2x80x9cOptimising performance in an xDSL line driverxe2x80x9d, Electronic design, Apr. 19, 1999.
It is well known to provide transformer based line drivers and an example of a typical hybrid arrangement of this general type is shown with reference to FIGS. 1 to 3. In the example shown, a simple series termination is employed.
There is no unified terminology for the whole circuit, which is used to convert of an analogue signal to the appropriate line level. In one convention it is referred to as a discrete AFE (but the amplifier can be integrated). Others sources call it a line interface circuit. The term line driver is sometimes use for the complete circuit and sometimes for only part of this circuit. Although there are differences in terminology, the principles of operation remain the same. Generally, the word line driver will be used in the following when describing the invention and when used in the attached claims and it should be understood that a line driver means the circuitry that performs conversion of the analogue signal to the appropriate line level, foresees a termination impedance and eventually provides power splitting between transmitted and received signal.
FIG. 1 is a schematic block diagram of a conventional line driver comprising an amplifier 2 feeding a hybrid circuit 1 with a termination impedance 10, the hybrid circuit being for transmitting signals to and receiving signals from a transmission line 9. The hybrid circuit is assumed almost loss free. The hybrid circuit 1 has an input 11 and an output 12 to receiver circuitry. The impedance of the hybrid circuit 1 is matched to that (Zref) of the line 9. If the output voltage of the amplifier 2 is m.V volts and the power consumed by the circuit is P=2xc3x97V (volts)xc3x97I (amps), then the current entering the hybrid circuit 1 is 2.I/m amps. The power consumed in the line 9 is Ixc3x97V. The impedance of the termination load is selected as k2Zref and the current in the termination impedance is I/k. The power consumed in the termination load 10 is I/kxc3x97Vxc3x97k=Ixc3x97V, i.e. 50% of the power is lost in the termination impedance. FIGS. 2 and 3 show a conventional circuit of this type, whereby FIG. 3 shows the voltages and currents present in the circuit. An operational amplifier 2 has an input of VNE/G (VNE refers to a near end voltage) where G is the gain represented by the line signal/input signal. The output of the operational amplifier 2 is connected to the centre-tapping of a primary winding 4,5 of a splitting transformer 3 (1:1:N). A first primary winding 4 of the transformer 1:1:N (3) is terminated in known fashion to ground for load matching to the line 9 via a termination resistance R (7), giving rise to a termination impedance Zref.
A second primary winding 5 of the transformer 1:1:N (3) is connected to the line 9, represented here by a load 8 in the form of an equivalent resistance Rload. For transmission, a near end signal having an amplitude VNE is placed on the load Rload, while during reception the circuit receives a far end signal VFE from the line at the same point. The symmetry in the operation of the circuit can be seen clearly as the voltages and currents in the termination load and in the line are the same.
The input power to the hybrid circuit is represented by P=2.V.I and it will be apparent to the skilled person that the power dissipated in the termination impedance Zref in this kind of arrangement is typically P/2, which means of course that half of the power generated is dissipated in the line driver itself.
One arrangement which improves the prior art approach of FIGS. 1 to 3 is shown in EP0901221, the disclosure of which is incorporated herein by way of reference and to which the skilled person is referred for a better understanding of known systems of this type. The arrangement of EP0901221 is suitable for matching a line driver with a predetermined load impedance, but with reduced power consumption when compared with the arrangement of FIGS. 1 to 3.
Although an improvement over the basic line driver, the arrangement of EP0901221 still requires a conventionally highly powered operational amplifier (op-amp) for its successful implementation and line driver power consumption remains at what might still be considered to be a high level. There continues to exist, therefore, a need for line driver arrangements that consume less power than some of those available to date.
It is an object of the present invention to provide an improved line driver arrangement.
It is also an object of the present invention to provide an improved method of operating a line driver.
It is also an object of the present invention to provide an improved line driver arrangement and method of operating the same which results in less wasted power dissipation.
Accordingly, the present invention provides a line driver for use with an associated load such as a transmission line, the line driver comprising a hybrid circuit, a signal portion and a termination portion connected to an output of the hybrid circuit for load matching, the termination portion including a termination impedance which is supplied in use by the hybrid circuit with a termination current, wherein said termination portion further comprises a back termination transformer, a first winding of the transformer being coupled in series with said termination resistance, a second winding of the transformer being coupled in parallel with said termination impedance so that a portion of the voltage dropped across the termination portion is dropped across the first winding to thereby cause the power dissipated in said termination impedance to be less than the power available to the signal transmission portion for signal transmission to the load, e.g. to an associated transmission line. To provide input impedance matching to the associated load (transmission line), a feedback circuit is provided to feedback current from the signal portion to an input of the hybrid circuit.
The present invention may also provide a line driver for use with an associated load such as a transmission line comprising a termination portion which includes a termination impedance and which is supplied in use by an associated amplifier, preferably in the form of an operational amplifier (op-amp), with a termination current for load matching, wherein said termination portion further comprises a back-termination transformer which is coupled with said termination impedance so that a current is injected into the termination portion via a winding of the back termination transformer, whereby the power dissipated in said termination impedance is less than the power available to a signal transmission portion for signal transmission to the associated load, e.g. to the associated transmission line.
The power dissipated in the termination portion compared to the power available in the load (transmission line) is one of at least 10% less, at least 20% less, at least 30% less, at least 40% less, or at least 50% less,
A back termination current may be induced in a winding of said back-termination transformer and may be fed back towards the output of said amplifier, whereby a portion of said termination current is recovered. The output of said amplifier may be reduced in proportion to said recovered current.
The impedance of a component of said termination portion may be scaled with respect to at least one of a turns ratio of a line transformer (1:1:N) and the impedance of said associated load (transmission line).
The value of said termination impedance may be lower than a characteristic impedance of said associated load (transmission line) by a factor in proportion to the turns ratio of said back termination transformer. A turns ratio of said back-termination transformer may be between 0 and 1.
During a receive operation, a portion of a current derived from a far end voltage may be fed back into an input of said amplifier in a proportion which is arranged to control the output of said amplifier in such a manner that correct input impedance matching from said termination portion is ensured during signal reception.
The receive operation feedback may be supplied from a node where the voltage level is substantially independent of the transmitted signal in a preferred but not mandatory aspect of the present invention, whereby a back-termination is implemented in such a manner that gain and stability during a transmit operation are substantially unaffected by said feedback.
Said termination impedance may be coupled in series with a primary winding of said back-termination transformer. Said termination impedance may be coupled in series with a secondary winding of said back-termination transformer. The value of said termination impedance may be determined in accordance with the equation Rterm=Rload. (1xe2x88x92xcex1), in which Rload is the impedance of the load (e.g. a line impedance) and xcex1 is a turns ratio of said back-termination transformer. Said amplifier is preferable built with an operational amplifier and it has a gain (A) in accordance with the equation A=2(1xe2x88x92xcex1)/(2xe2x88x92xcex1), in which xcex1 is the turns ratio of said back-termination transformer.
Said hybrid circuit may be adapted such that the voltages at the termination port are scaled by a factor k, the currents by a factor 1/k, the associated turns ratios by a factor k and the impedances by a factor k2. The hybrid circuit may be adapted such that the voltage at the hybrid input, which is connected to said amplifier, is scaled by a factor m, the currents by a factor 1/m, the associated turns ratios by a factor m and the impedances by a factor m2.
Said back-termination transformer may comprise an auto-transformer.
Said back-termination transformer may be included in a feedback loop which provides feedback to the input side of said amplifier, said amplifier being implemented using an operational amplifier which operates off a low voltage supply, such as for example a voltage in the region of +/xe2x88x925 or 6V.
A secondary winding of said back-termination transformer may be serially coupled with a primary winding of a line matching transformer of said line driver. Said line matching transformer may comprise a splitting transformer, a first primary winding of which is coupled in series with said secondary winding of said back-termination transformer and a second primary winding of which is coupled to a load. A secondary winding of said line-matching transformer might be coupled to receiving circuitry. Said line matching transformer may be operated as a common-mode choke.
Said back termination transformer may be integrated with a transmission line transformer. In this case a complete front end can be implemented using only two transformers, with the back-termination transformer incorporated in such a transmit-transformer.
The nominal output power of said line driver may be in the region of (2xe2x88x92xcex1).VNE2/Rterm, in which: xcex1 is the turns ratio of said back-termination transformer (1:xcex1); VNE is the near-end voltage of said line driver; and Rterm is the value of said termination resistance.
In an xDSL network, said line driver may be implemented at a customer premises (CPE).
The present invention provides a method of operating a line driver for use with an associated load such as a transmission line, said line driver comprising, a hybrid circuit, a signal portion and a termination portion which includes a termination impedance and a back-termination transformer which is coupled with said termination impedance, the method including dropping across a winding of the back transformer a portion of a voltage dropped across the termination portion to thereby cause the power dissipated in said termination impedance to be less than the power available to the signal transmission portion for signal transmission to the load, e.g. to the associated transmission line. To provide input impedance matching to the associated load (transmission line), a feedback circuit is fed back current from the signal portion to an input of the hybrid circuit.
The present invention also provides a method of operating a line driver for use with an associated load (transmission line), said line driver comprising a termination portion which includes a termination impedance and a back-termination transformer which is coupled with said termination impedance, the method including recovering with said back-termination transformer a portion of a termination current and feeding the recovered current back in to a current supply from which at least a portion of said termination current is derived.
The method may include dissipating in said termination impedance less power than is dissipated in an associated load impedance (line impedance).
The method may include feeding back into a current generator during a receive operation an impedance synthesising current derived from a far-end voltage and thereby synthesising during a said receive operation a preferred line matching impedance in said termination portion.
The method may include feeding back said impedance synthesising current from a node whose current level is independent of transmitted signals.
The present invention also provides a modem suitable for broadband communication, preferably an xDSL modem, including a line driver according to the apparatus of the invention and/or including a line driver which operates in accordance with the method of the invention.